1. Field of the Invention
The present invention relates to a method for improving productivity of L-methionine and organic acid.
2. Description of the Related Art
Methionine is one of the essential amino acids in the body, and has been widely used as an animal feed and food additive, as well as a component of medical aqueous solutions and other raw material for medicinal products. Methionine acts as a precursor of choline (lecithin) and creatine, and is also used as a raw material for the synthesis of cysteine and taurine. In addition, it functions as a sulfur donor. S-adenosyl-methionine is derived from L-methionine and serves as a methyl donor in the body, and it is involved in the synthesis of various neurotransmitters in the brain. Methionine and/or S-adenosyl-L-methionine (SAM) is/are also found to prevent lipid accumulation in the liver and arteries and to be effective for the treatment of depression, inflammation, liver diseases and muscle pain (Jeon B R et al., J. Hepatol., 2001 March; 34(3): 395-401).
For the chemical synthesis of methionine, L-methionine is produced through the hydrolysis of 5-(β-methylmercaptoethyl)-hydantoin. However, the chemically synthesized methionine is disadvantageously present in a mixture of L- and D-forms. Therefore, the present inventors developed a biological method for selectively synthesizing L-methionine, and have already applied for a patent (WO 2008/103432). The method, is termed briefly as “a two-step process”, comprises the fermentative production of an L-methionine precursor and the enzymatic conversion of the L-methionine precursor to L-methionine. The L-methionine precursor preferably includes O-acetyl homoserine and O-succinyl homoserine. The two-step process is evaluated in terms of having overcome the problems from which the conventional methods suffer, such as sulfide toxicity, feedback regulation of strain by methionine and SAMe, and degradation of intermediates by cystathionine gamma synthase, O-succinyl homoserine sulfhydrylase and O-acetyl homoserine sulfhydrylase. Also, compared to the conventional chemical synthesis method of producing D- and L-methionine, the two-step process has the advantage of being selective for L-methionine only, with the concomitant production of organic acids, more particularly, succinic acid and acetic acid as useful by-products. The succinic acid is used as a raw material of paints, cosmetics, or medicinal products, and acetic acid is very useful in industrial fields, including preparation of vinyl acetic acid, staining, medicinal products such as aspirin, and photographic fixing solutions.
In the enzymatic conversion reaction of the two-step process, enzymes which have the activities of cystathionine gamma synthase, O-succinyl homoserine sulfhydrylase and O-acetyl homoserine sulfhydrylase are used, and O-acetyl homoserine or O-succinyl homoserine as the L-methionine precursor is mixed with methyl mercaptan to produce L-methionine and an organic acid by enzymatic reaction.
Methyl mercaptan exists as a gas at room temperature, and is slightly soluble in water, and has a high solubility in alkaline solutions. The enzymatic conversion reaction for L-methionine production occurs in an aqueous solution. Thus, if methyl mercaptan has a more improved solubility in the aqueous solution, it is expected to greatly increase methionine productivity.
Considering the above problem, the present inventors have made an effort to increase the solubility of methyl mercaptan in the enzymatic conversion reaction for maximization of L-methionine production. As a result, they found that a mixture of methyl mercaptan and dimethyl sulfide mixed at a appropriate ratio can improve the conversion rate of L-methionine and organic acid from L-methionine precursor, and thus L-methionine can be produced in a high yield, compared to the conventional methods, thereby completing the present invention.